These are OK. If I comment out MYSQL_SELECT_CLAUSE and let authmysql build
its own query, then everything is hunky-dory, the query works, and shows up
in the mysql log, so it's not connectivity.

There is something broken with authmysql as far as I can tell, though I'm
not sure what it is. Q.v. my postings of about a month ago, relating to
similar problems, on which I never received feedback:

My problem is similar, but authmysql fails even when the user is an existing
user, and I get the "Service Temporarily Unavailable" response. Watching
the mysql logs indicates than no query is made.

This is not so much a problem with my customer's new courier installation,
since the authentication there is fairly simple and probably for only one
domain name, but this problem effectively prevents me from upgrading courier
on my on server since I use a fairly complex MYSQL_SELECT_CLAUSE for
authentication and I'd have several dozen people coming after me with ropes
if I broke their company email feeds!